1. Relative Humidity
The most commonly used measure of humidity is relative humidity. Relative humidity can be simply defined as the amount of water in the air relative to the saturation amount the air can hold at a given temperature multiplied by 100. Air with a relative humidity of 50% contains a half of the water vapour it could hold at a particular temperature.

2. Temperature & Pressure

3. Measuring Relative Humidity
Humidity can be measured using a variety of instruments. Relative humidity is often determined using a sling psychrometer (wet and dry bulb):
Stephenson’s screen

4. Diurnal Humidity Changes

5. Föhn Wind (or Föhn effect)

6. Cloud Formation Triggers

7. Thunderstorm Development
During the cumulus stage, strong updrafts act to build the storm. The mature stage is marked by heavy precipitation and cool downdrafts in part of the storm. When the warm updrafts disappear completely, precipitation becomes light and the cloud begins to evaporate

8. St Elmo’s Fire
St. Elmo's fire is a spark discharge from soaring buildings. It is generated by a high voltage between the ground and the air. St. Elmo´s fire has been observed among others from steeples, masts, mountain tops and barbed wire fences. It is very rare. When you see St. Elmo´s fire near yourself, there is a high danger of being struck by lightning. The high voltage can also show up by your hair standing on end.
Although this may look "funny", you must immediately leave your position as a flash of lightning is imminent. It seems that there are just very few photographs of St. Elmo's fire. When there is thunderstorm, just watch out for sparks going upward from steeples and TV-antennas near you.

9. Cloud Types

10. TAF Code (Terminal Airfield Forecast)
This TAF example is from Calgary International Airport, Calgary, Alberta, and was released on October 19, 2006 at 2038 UTC:
TAF CYYC Z KT P6SM SCT020 OVC080 TEMPO 2203 P6SM -SHRA BECMG KT FM0300Z 32010KT P6SM SCT007 BKN060 FM0600Z 33015KT P6SM SCT010 BKN040 TEMPO SM -RASN BR OVC010 FM1200Z 34015G25KT P6SM SCT010 OVC030 TEMPO SM -SHSN OVC010 RMK NXT FCST BY 00Z=
TAF indicates that the following is a terminal area forecast.
CYYC indicates that the report came from Calgary International Airport.
192038Z indicates that the report was issued at 2038 UTC on the 19th of the month.
indicates that the report is valid from 2100 UTC on the 19th until 1800 UTC on the following day.
17008KT indicates that the forecast wind at 2100 UTC will initially be from 170 degrees at 8 knots.
P6SM indicates that visibility is forecast to be at least six statute miles. Forecast visibility of six miles or more is always referred to as P6SM. The P stands for Plus.
SCT020 OVC080 indicates that clouds are forecast to be scattered at 2000 feet and overcast at 8000 feet.
TEMPO 2203 P6SM -SHRA indicates that between 2200 and 0300 there may be at times light rain showers with visibility of at least six statute miles.
BECMG KT indicates that a wind shift to 240 degrees at 7 knots is forecast to occur between 2200 and 2300 UTC.
FM0300Z 32010KT P6SM SCT007 BKN060 indicates that beginning at 0300 UTC the wind will be from 320 degrees at 10 knots, visibility will be at least six statute miles, and clouds will be scattered at 700 feet and broken at 6000 feet.
FM0600Z 33015KT P6SM SCT010 BKN040 TEMPO SM -RASN BR OVC010 indicates that beginning at 0600 UTC the wind will be from 330 degrees at 15 knots, visibility will be at least six statute miles, and clouds will be scattered at 1000 feet and broken at 4000 feet. There is forecast to be at times between 0600 and 1200 hours visibility at 5 statute miles, light rain and snow, and mist with an overcast layer of cloud at 1000 feet.
FM1200Z 34015G25KT P6SM SCT010 OVC030 TEMPO SM -SHSN OVC010 indicates that beginning at 1200 UTC the wind will be from 340 degrees at 15 knots gusting to 25 knots, visibility will be at least six statute miles, and clouds will be scattered at 1000 feet and overcast at 3000 feet. There is also forecast to be at times between 1200 and 1800 hours visibility of two statute miles, light snow showers, and an overcast layer of cloud at 1000 feet.
RMK NXT FCST BY 00Z indicates that the next forecast will be issued by 0000 UTC.

11. METAR (Meteorological Actual Report)
The following is an example METAR from Burgas Airport in Burgas, Bulgaria, and was taken on February 4, 2005 at 16:00 Coordinated Universal Time (UTC).
METAR LBBG Z 12003MPS 310V R04/P1500N R22/P1500U +SN BKN022 OVC050 M04/M07 Q1020 NOSIG 9949//91=
METAR indicates that the following is a standard hourly observation.
LBBG is the ICAO airport code for Burgas Airport.
041600Z indicates the day of the month is the 4th and the time of day is 1600 Zulu time, 4:00PM Greenwich Mean Time, or 6:00PM Eastern European Time.
12003MPS indicates the wind direction is from 120 degrees true (east-southeast) at a speed of 3 meters per second.
310V290 indicates the wind direction is varying from 310 degrees true (northwest) through 120 degrees true (east-southeast) to 290 degrees true (west-northwest).
1400 indicates the prevailing visibility is 1400 metres.
R04/P1500N indicates the Runway visual range (RVR) along Runway 04 is 1500 meters and not changing significantly.
R22/P1500U indicates RVR along Runway 22 is 1500 meters and rising.
+SN indicates snow is falling at a heavy intensity.
BKN022 indicates a broken cloud layer at 2,200 feet above ground level (agl).
OVC050 indicates an unbroken cloud layer (overcast) at 5,000 feet above ground level.
M04/M07 indicates the temperature is minus 4 degrees Celsius and the dewpoint is minus 7 degrees Celsius.
Q1020 indicates the current barometric pressure extrapolated to sea level is 1020 millibars.
NOSIG is an example of a TREND forecast which is appended to METARs at stations while a forecaster is on watch. NOSIG means that no significant change is expected to the reported conditions within the next 2 hours.
9949//91 indicates runway status. Format: abcdefgh ab=runway heading, c=nature of coating (4=dry snow), d=surface covered in percent (9= % of rwy covered), ef=thickness of coating in millimeters (// stands for either not measurable or not affecting usage of rwy) gh=braking index (91=bad braking index i.e grip on rwy)
CAVOK indicates Ceiling And Visibility OKay (no cloud below 5000 feet, a visibility of 6 Statute Miles or more and no precipitation, thunderstorms, shallow fog, or low drifting snow)
= indicates the end of the METAR report