WEATHER FOR BUSINESS

We supply weather data to businesses

Free Weather Plugin

Add this forecast to your website



Local Weather

Zanzibar 14 Day Long Range Forecast

14 Day weather providing an extended long range forecast outlook for Zanzibar

Tanzania

Long range weather outlook for Zanzibar includes 14 day forecast summary:

Over the next 14 days the Zanzibar forecast suggests the average daytime maximum temperature will be around 30°C, with a high for the two weeks of 31°C expected on the afternoon of Sunday 12th. The mean minimum temperature will be 24°C, dipping to its lowest on the morning of Tuesday 7th at 23°C. Expect the following 14 days ahead to have some days seeing a little precipitation and some days with rain. Predictions are Tuesday 7th will have the most precipitation with an accumulation of around 11.0mm. On the whole winds are likely to be moderate.

DayLocal TimeDirSpeed/
Gust
Temp/
Feels Like
HumidityPressureCloud AmountPrecip AmountWeather
Fri 03
May
18:00

SSW
SSW

14 / 17 mph 87 / 98°F 78% 1009 mb
Low level cloud: 18%
Mid level cloud: 31%
High level cloud: 32%
Most Significant Cloud Type: low
32%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
21:00

SSW
SSW

15 / 17 mph 83 / 91°F 77% 1011 mb
Low level cloud: 15%
Mid level cloud: 21%
High level cloud: 54%
Most Significant Cloud Type: low
28%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
Sat 04
May
00:00

SW
SW

15 / 22 mph 79 / 83°F 77% 1011 mb
Low level cloud: 24%
Mid level cloud: 16%
High level cloud: 91%
Most Significant Cloud Type: low
40%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
03:00

SSW
SSW

15 / 21 mph 78 / 81°F 77% 1010 mb
Low level cloud: 82%
Mid level cloud: 26%
High level cloud: 32%
Most Significant Cloud Type: low
94%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
06:00

SW
SW

13 / 20 mph 78 / 81°F 75% 1012 mb
Low level cloud: 65%
Mid level cloud: 27%
High level cloud: 3%
Most Significant Cloud Type: low
75%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
09:00

SW
SW

15 / 19 mph 81 / 83°F 73% 1013 mb
Low level cloud: 66%
Mid level cloud: 16%
High level cloud: 6%
Most Significant Cloud Type: low
72%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
12:00

SSW
SSW

14 / 17 mph 82 / 87°F 68% 1013 mb
Low level cloud: 88%
Mid level cloud: 33%
High level cloud: 20%
Most Significant Cloud Type: low
100%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
15:00

SSW
SSW

14 / 17 mph 83 / 87°F 68% 1010 mb
Low level cloud: 61%
Mid level cloud: 45%
High level cloud: 0%
Most Significant Cloud Type: low
76%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
18:00

SSW
SSW

16 / 23 mph 80 / 82°F 75% 1012 mb
Low level cloud: 20%
Mid level cloud: 22%
High level cloud: 0%
Most Significant Cloud Type: low
27%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
21:00

SSW
SSW

15 / 21 mph 76 / 79°F 77% 1014 mb
Low level cloud: 28%
Mid level cloud: 17%
High level cloud: 0%
Most Significant Cloud Type: low
34%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
Sun 05
May
00:00

SSW
SSW

14 / 16 mph 77 / 81°F 82% 1014 mb
Low level cloud: 38%
Mid level cloud: 45%
High level cloud: 0%
Most Significant Cloud Type: low
53%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
03:00

SSW
SSW

14 / 17 mph 75 / 79°F 82% 1012 mb
Low level cloud: 79%
Mid level cloud: 45%
High level cloud: 0%
Most Significant Cloud Type: low
94%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
06:00

SW
SW

13 / 19 mph 74 / 78°F 82% 1013 mb
Low level cloud: 81%
Mid level cloud: 58%
High level cloud: 1%
Most Significant Cloud Type: low
100%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
09:00

SW
SW

12 / 14 mph 77 / 82°F 84% 1015 mb
Low level cloud: 84%
Mid level cloud: 92%
High level cloud: 70%
Most Significant Cloud Type: low
100%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
12:00

SSW
SSW

11 / 14 mph 81 / 88°F 87% 1014 mb
Low level cloud: 74%
Mid level cloud: 84%
High level cloud: 50%
Most Significant Cloud Type: low
100%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
15:00

SSW
SSW

9 / 14 mph 84 / 93°F 90% 1012 mb
Low level cloud: 26%
Mid level cloud: 48%
High level cloud: 98%
Most Significant Cloud Type: low
55%
Expect 0.1 In of precipitation over the past 3 hours.
0.1 In
18:00

S
S

9 / 11 mph 82 / 90°F 89% 1013 mb
Low level cloud: 62%
Mid level cloud: 64%
High level cloud: 80%
Most Significant Cloud Type: low
93%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
21:00

SSE
SSE

11 / 16 mph 79 / 85°F 93% 1014 mb
Low level cloud: 87%
Mid level cloud: 49%
High level cloud: 100%
Most Significant Cloud Type: low
100%
Expect 0.1 In of precipitation over the past 3 hours.
0.1 In
Mon 06
May
00:00

S
S

7 / 12 mph 77 / 82°F 94% 1014 mb
Low level cloud: 80%
Mid level cloud: 68%
High level cloud: 99%
Most Significant Cloud Type: low
100%
Expect 0.1 In of precipitation over the past 3 hours.
0.1 In
03:00

SSW
SSW

11 / 16 mph 75 / 80°F 94% 1012 mb
Low level cloud: 88%
Mid level cloud: 71%
High level cloud: 99%
Most Significant Cloud Type: low
100%
Expect 0.2 In of precipitation over the past 3 hours.
0.2 In
06:00

SSW
SSW

9 / 15 mph 74 / 78°F 92% 1013 mb
Low level cloud: 52%
Mid level cloud: 54%
High level cloud: 100%
Most Significant Cloud Type: low
82%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
09:00

SSW
SSW

8 / 11 mph 76 / 81°F 91% 1015 mb
Low level cloud: 26%
Mid level cloud: 86%
High level cloud: 43%
Most Significant Cloud Type: medium
100%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
12:00

SSW
SSW

7 / 9 mph 80 / 84°F 85% 1015 mb
Low level cloud: 42%
Mid level cloud: 74%
High level cloud: 98%
Most Significant Cloud Type: low
79%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
15:00

S
S

5 / 6 mph 83 / 85°F 78% 1012 mb
Low level cloud: 24%
Mid level cloud: 48%
High level cloud: 100%
Most Significant Cloud Type: low
52%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
18:00

SSE
SSE

4 / 5 mph 83 / 86°F 81% 1013 mb
Low level cloud: 2%
Mid level cloud: 44%
High level cloud: 33%
Most Significant Cloud Type: medium
49%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
21:00

SSE
SSE

4 / 7 mph 78 / 83°F 88% 1014 mb
Low level cloud: 19%
Mid level cloud: 60%
High level cloud: 83%
Most Significant Cloud Type: medium
100%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
Tue 07
May
00:00

S
S

4 / 7 mph 77 / 82°F 91% 1014 mb
Low level cloud: 7%
Mid level cloud: 45%
High level cloud: 41%
Most Significant Cloud Type: medium
67%
Expect 0.1 In of precipitation over the past 3 hours.
0.1 In
03:00

SSW
SSW

4 / 8 mph 75 / 79°F 92% 1013 mb
Low level cloud: 7%
Mid level cloud: 56%
High level cloud: 96%
Most Significant Cloud Type: medium
80%
Expect 0.1 In of precipitation over the past 3 hours.
0.1 In
06:00

SSW
SSW

7 / 13 mph 74 / 78°F 92% 1014 mb
Low level cloud: 22%
Mid level cloud: 49%
High level cloud: 37%
Most Significant Cloud Type: low
43%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
09:00

SSW
SSW

9 / 13 mph 78 / 81°F 92% 1016 mb
Low level cloud: 28%
Mid level cloud: 51%
High level cloud: 87%
Most Significant Cloud Type: low
56%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
12:00

SSW
SSW

10 / 11 mph 81 / 85°F 91% 1015 mb
Low level cloud: 63%
Mid level cloud: 26%
High level cloud: 10%
Most Significant Cloud Type: low
73%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
15:00

SSE
SSE

9 / 11 mph 82 / 87°F 85% 1013 mb
Low level cloud: 17%
Mid level cloud: 38%
High level cloud: 21%
Most Significant Cloud Type: low
33%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
18:00

SSE
SSE

10 / 15 mph 80 / 86°F 90% 1014 mb
Low level cloud: 6%
Mid level cloud: 81%
High level cloud: 16%
Most Significant Cloud Type: medium
98%
Expect 0.1 In of precipitation over the past 3 hours.
0.1 In
21:00

S
S

9 / 16 mph 78 / 83°F 90% 1015 mb
Low level cloud: 23%
Mid level cloud: 44%
High level cloud: 14%
Most Significant Cloud Type: low
40%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
Wed 08
May
00:00

S
S

9 / 15 mph 77 / 81°F 89% 1015 mb
Low level cloud: 39%
Mid level cloud: 7%
High level cloud: 45%
Most Significant Cloud Type: low
46%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
03:00

S
S

10 / 16 mph 76 / 81°F 92% 1013 mb
Low level cloud: 11%
Mid level cloud: 17%
High level cloud: 6%
Most Significant Cloud Type: low
18%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
06:00

S
S

11 / 18 mph 76 / 80°F 92% 1014 mb
Low level cloud: 24%
Mid level cloud: 6%
High level cloud: 3%
Most Significant Cloud Type: low
26%
Expect 0.1 In of precipitation over the past 3 hours.
0.1 In
09:00

SSW
SSW

11 / 14 mph 80 / 84°F 83% 1016 mb
Low level cloud: 33%
Mid level cloud: 6%
High level cloud: 4%
Most Significant Cloud Type: low
35%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
12:00

SSW
SSW

11 / 13 mph 85 / 89°F 67% 1014 mb
Low level cloud: 48%
Mid level cloud: 6%
High level cloud: 7%
Most Significant Cloud Type: low
50%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
15:00

SSE
SSE

9 / 12 mph 87 / 90°F 63% 1012 mb
Low level cloud: 5%
Mid level cloud: 14%
High level cloud: 3%
Most Significant Cloud Type: low
9%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
18:00

SSE
SSE

10 / 17 mph 82 / 85°F 80% 1013 mb
Low level cloud: 2%
Mid level cloud: 46%
High level cloud: 3%
Most Significant Cloud Type: medium
51%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
21:00

S
S

10 / 15 mph 79 / 84°F 80% 1014 mb
Low level cloud: 1%
Mid level cloud: 7%
High level cloud: 1%
Most Significant Cloud Type: medium
8%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
Thu 09
May
00:00

S
S

9 / 14 mph 78 / 83°F 87% 1014 mb
Low level cloud: 0%
Mid level cloud: 12%
High level cloud: 0%
Most Significant Cloud Type: medium
11%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
03:00

S
S

9 / 16 mph 76 / 81°F 89% 1012 mb
Low level cloud: 1%
Mid level cloud: 22%
High level cloud: 0%
Most Significant Cloud Type: medium
23%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
06:00

SSW
SSW

9 / 15 mph 75 / 79°F 87% 1013 mb
Low level cloud: 8%
Mid level cloud: 42%
High level cloud: 0%
Most Significant Cloud Type: medium
64%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
09:00

SW
SW

11 / 13 mph 80 / 83°F 76% 1015 mb
Low level cloud: 16%
Mid level cloud: 31%
High level cloud: 0%
Most Significant Cloud Type: low
26%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
12:00

SSW
SSW

10 / 11 mph 84 / 87°F 71% 1013 mb
Low level cloud: 44%
Mid level cloud: 15%
High level cloud: 2%
Most Significant Cloud Type: low
49%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
15:00

SSE
SSE

9 / 11 mph 86 / 90°F 65% 1010 mb
Low level cloud: 19%
Mid level cloud: 19%
High level cloud: 0%
Most Significant Cloud Type: low
26%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
18:00

SSE
SSE

12 / 18 mph 82 / 87°F 79% 1011 mb
Low level cloud: 1%
Mid level cloud: 11%
High level cloud: 6%
Most Significant Cloud Type: medium
14%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
21:00

SSE
SSE

11 / 17 mph 79 / 85°F 81% 1014 mb
Low level cloud: 16%
Mid level cloud: 12%
High level cloud: 11%
Most Significant Cloud Type: low
21%
Expect 0.0 In of precipitation over the past 3 hours.
0.0 In
DayLocal TimeDirSpeed/
Gust
Temp/
Feels Like
HumidityPressureCloud AmountPrecip AmountWeather
Fri 10
May
03:00

S
S

9 / 18 mph 78 / 83°F 88% 1012 mb
Low level cloud: 2%
Mid level cloud: 44%
High level cloud: 4%
Most Significant Cloud Type: medium
49%
0.0 In
09:00

SSW
SSW

11 / 22 mph 77 / 81°F 90% 1014 mb
Low level cloud: 66%
Mid level cloud: 29%
High level cloud: 0%
Most Significant Cloud Type: low
76%
0.0 In
15:00

SSE
SSE

12 / 26 mph 84 / 91°F 72% 1011 mb
Low level cloud: 15%
Mid level cloud: 27%
High level cloud: 0%
Most Significant Cloud Type: low
24%
0.0 In
21:00

SSE
SSE

11 / 23 mph 79 / 85°F 91% 1014 mb
Low level cloud: 8%
Mid level cloud: 17%
High level cloud: 0%
Most Significant Cloud Type: low
14%
0.0 In
Sat 11
May
03:00

SSE
SSE

10 / 21 mph 73 / 77°F 92% 1013 mb
Low level cloud: 1%
Mid level cloud: 28%
High level cloud: 0%
Most Significant Cloud Type: medium
31%
0.0 In
09:00

SSW
SSW

9 / 19 mph 79 / 84°F 84% 1015 mb
Low level cloud: 10%
Mid level cloud: 11%
High level cloud: 0%
Most Significant Cloud Type: low
14%
0.0 In
15:00

SSE
SSE

6 / 13 mph 88 / 96°F 62% 1011 mb
Low level cloud: 44%
Mid level cloud: 23%
High level cloud: 0%
Most Significant Cloud Type: low
51%
0.0 In
21:00

SSE
SSE

10 / 20 mph 84 / 93°F 81% 1014 mb
Low level cloud: 0%
Mid level cloud: 9%
High level cloud: 15%
Most Significant Cloud Type: medium
9%
0.0 In
Sun 12
May
03:00

S
S

8 / 17 mph 76 / 80°F 86% 1012 mb
Low level cloud: 16%
Mid level cloud: 5%
High level cloud: 0%
Most Significant Cloud Type: low
17%
0.0 In
09:00

SW
SW

7 / 15 mph 79 / 84°F 80% 1015 mb
Low level cloud: 69%
Mid level cloud: 8%
High level cloud: 0%
Most Significant Cloud Type: low
71%
0.0 In
15:00

SSE
SSE

7 / 15 mph 91 / 104°F 62% 1011 mb
Low level cloud: 28%
Mid level cloud: 4%
High level cloud: 0%
Most Significant Cloud Type: low
28%
0.0 In
21:00

SSE
SSE

9 / 20 mph 79 / 84°F 79% 1013 mb
Low level cloud: 0%
Mid level cloud: 11%
High level cloud: 0%
Most Significant Cloud Type: medium
10%
0.0 In
Mon 13
May
03:00

S
S

8 / 17 mph 76 / 80°F 90% 1012 mb
Low level cloud: 5%
Mid level cloud: 16%
High level cloud: 0%
Most Significant Cloud Type: medium
28%
0.0 In
09:00

SSW
SSW

7 / 15 mph 80 / 86°F 79% 1014 mb
Low level cloud: 50%
Mid level cloud: 28%
High level cloud: 0%
Most Significant Cloud Type: low
59%
0.0 In
15:00

SSE
SSE

10 / 21 mph 90 / 100°F 62% 1011 mb
Low level cloud: 11%
Mid level cloud: 13%
High level cloud: 0%
Most Significant Cloud Type: low
15%
0.0 In
21:00

SE
SE

11 / 23 mph 78 / 82°F 79% 1012 mb
Low level cloud: 0%
Mid level cloud: 16%
High level cloud: 0%
Most Significant Cloud Type: medium
15%
0.0 In
Tue 14
May
03:00

S
S

8 / 16 mph 82 / 89°F 82% 1011 mb
Low level cloud: 1%
Mid level cloud: 7%
High level cloud: 0%
Most Significant Cloud Type: medium
10%
0.0 In
09:00

SSW
SSW

8 / 16 mph 79 / 84°F 81% 1013 mb
Low level cloud: 45%
Mid level cloud: 17%
High level cloud: 0%
Most Significant Cloud Type: low
50%
0.1 In
15:00

S
S

9 / 18 mph 87 / 93°F 60% 1010 mb
Low level cloud: 44%
Mid level cloud: 11%
High level cloud: 0%
Most Significant Cloud Type: low
48%
0.0 In
21:00

SSE
SSE

11 / 23 mph 78 / 82°F 79% 1012 mb
Low level cloud: 0%
Mid level cloud: 5%
High level cloud: 0%
Most Significant Cloud Type: medium
4%
0.0 In
Wed 15
May
03:00

S
S

10 / 21 mph 78 / 81°F 72% 1012 mb
Low level cloud: 0%
Mid level cloud: 2%
High level cloud: 0%
Most Significant Cloud Type: medium
1%
0.0 In
09:00

SW
SW

7 / 14 mph 80 / 85°F 81% 1013 mb
Low level cloud: 59%
Mid level cloud: 4%
High level cloud: 0%
Most Significant Cloud Type: low
60%
0.0 In
15:00

S
S

8 / 17 mph 86 / 92°F 59% 1010 mb
Low level cloud: 23%
Mid level cloud: 7%
High level cloud: 0%
Most Significant Cloud Type: low
25%
0.0 In
21:00

SSE
SSE

11 / 22 mph 78 / 83°F 79% 1012 mb
Low level cloud: 0%
Mid level cloud: 1%
High level cloud: 0%
Most Significant Cloud Type: medium
1%
0.0 In
Thu 16
May
03:00

SSW
SSW

8 / 17 mph 75 / 79°F 81% 1011 mb
Low level cloud: 1%
Mid level cloud: 41%
High level cloud: 0%
Most Significant Cloud Type: medium
43%
0.0 In
09:00

SW
SW

7 / 14 mph 80 / 84°F 79% 1014 mb
Low level cloud: 47%
Mid level cloud: 2%
High level cloud: 0%
Most Significant Cloud Type: low
47%
0.0 In
15:00

S
S

8 / 17 mph 89 / 96°F 59% 1010 mb
Low level cloud: 48%
Mid level cloud: 21%
High level cloud: 0%
Most Significant Cloud Type: low
55%
0.0 In
21:00

SSE
SSE

9 / 20 mph 79 / 84°F 81% 1013 mb
Low level cloud: 0%
Mid level cloud: 6%
High level cloud: 0%
Most Significant Cloud Type: medium
5%
0.0 In
Fri 17
May
03:00

S
S

11 / 22 mph 77 / 82°F 91% 1012 mb
Low level cloud: 6%
Mid level cloud: 26%
High level cloud: 0%
Most Significant Cloud Type: medium
41%
0.1 In
09:00

SSW
SSW

8 / 16 mph 79 / 84°F 83% 1015 mb
Low level cloud: 58%
Mid level cloud: 6%
High level cloud: 0%
Most Significant Cloud Type: low
60%
0.0 In
15:00

SSE
SSE

15 / 31 mph 82 / 88°F 75% 1012 mb
Low level cloud: 19%
Mid level cloud: 28%
High level cloud: 0%
Most Significant Cloud Type: low
29%
0.2 In
21:00

SE
SE

8 / 17 mph 80 / 87°F 89% 1013 mb
Low level cloud: 3%
Mid level cloud: 3%
High level cloud: 0%
Most Significant Cloud Type: low
4%
0.0 In
Sat 18
May
03:00

S
S

10 / 21 mph 78 / 82°F 81% 1013 mb
Low level cloud: 20%
Mid level cloud: 28%
High level cloud: 0%
Most Significant Cloud Type: low
29%
0.0 In
09:00

SW
SW

6 / 13 mph 80 / 86°F 81% 1015 mb
Low level cloud: 70%
Mid level cloud: 5%
High level cloud: 0%
Most Significant Cloud Type: low
71%
0.0 In
15:00

SE
SE

13 / 28 mph 84 / 92°F 71% 1012 mb
Low level cloud: 31%
Mid level cloud: 19%
High level cloud: 0%
Most Significant Cloud Type: low
37%
0.1 In
21:00

SSE
SSE

10 / 22 mph 79 / 86°F 86% 1012 mb
Low level cloud: 2%
Mid level cloud: 6%
High level cloud: 0%
Most Significant Cloud Type: low
4%
0.0 In

The day label given represents the local day relative to the local time for the location you are looking at.
(More Info)

Day

This time is corrected for local time zones and where possible for daylight saving times.
(More Info)

Local Time

The wind direction we use on this page is the direction the wind is coming from, given in a 16 point compass format.
(More Info)

Dir

This refers to the sustained average wind speed, normally averaged over a period of 10 minutes for up to 3 hrs.
(More Info)

Speed/
Gust

Temperature from our forecast perspective are fairly well defined, they are what we would expect to measure in a standard meteorological screen (in other words, shaded and well ventilated) at 2 metres above ground level.
(More Info)

Temp/
Feels Like

The relative humidity is the percent of saturation humidity, generally calculated in relation to saturated vapour density.
(More Info)

Humidity

The value given is a total predicted for the previous 3 hrs and includes the time of the forecast being looked at.
(More Info)

Precip Amount

The total amount of cloud as a percentage is derived from looking at cloud cover throughout the atmosphere and estimating how these combine when looked at from the ground.
(More Info)

Cloud Amount

Here we try to capture the feel of the weather as an image – it will never be perfect but is useful for a quick overview.
(More Info)

Weather

When we measure for forecast air pressure we are normally doing this relative to a certain height and most commonly relative to Mean Sea Level.
(More Info)

Pressure